As personal computers have evolved, the processing power and available memory have both increased with each passing year. However, to allow for continuing use of existing software applications and hardware devices, current computers must take into consideration limitations inherent in earlier computer models and earlier external devices so as to provide compatibility. This is sometimes referred to as legacy support, support for legacy systems, support for legacy devices, etc.
An external bus supported by many current personal computers is the Universal Serial Bus (USB). Version 1.1 of the USB standard provides for low speed communication at 1.5 Mbps and full speed communication at 12 Mbps. For more information see Universal Serial Bus Specification, Revision 1.1 dated Sep. 23, 1998 available from USB Implementers Forum, Inc. of Portland, Oreg. A newer version of the USB standard, version 2.0, provides for high speed communication at 480 Mbps., as well as providing support for low and full speed communication. For more information see Universal Serial Bus Specification, Revision 2.0 dated Apr. 27, 2000 available from USB Implementers Forum, Inc. of Portland, Oreg. Because many existing external USB enabled devices such as digital cameras, printers, etc., adhere to the USB 1.1 standard, USB 2.0 takes this into consideration and allows for USB 1.1 devices to be used with USB 2.0 host controllers and hubs.
To accomplish the improvement in speed over USB 1.1, USB 2.0 defines micro-frames of 125 microseconds such that eight micro-frames comprise the 1 millisecond frames described in the USB 1.1 standard. As each frame includes several micro-frames under USB 2.0, each frame may contain several transactions. For isochronous and other kinds of periodic transfers which initiate from a USB 1.1 device, multiple transactions may be split and may be included in micro-frames in each of a plurality of successive frames. It is up to the USB 2.0 host controller and hub controller implementation to support split transactions.